An influence on plant growth by various strains of bacteria derived from the root zones of plants (the "rhizosphere") has been documented in several crops, including radish, potato and sugar beet. Both positive and negative growth effects mediated by rhizobacteria have been observed. Of particular interest are the plant growth-promoting rhizobacteria (PGPR), which are known to enhance plant growth in the aforementioned tuber plants, as well as in apple rootstocks, under field conditions. The following literature citations, which are hereby incorporated by reference, provide background information on known rhizobacterial effects on plant growth:
Kloepper & Schroth, "Plant Growth Promoting Rhizobacteria on Radishes," in PROC. 4th INT. CONF. PLANT PATH. BACT. ANGERS 879-82 (1978). PA1 Burr et al, "Increased Potato Yields by Treatment of Seed Pieces With Specific Strains of Pseudomonas fluorescens and P. putida," Phytopathology 68:1377-83 (1979). PA1 Kloepper et al, "Effects of Rhizosphere Colonization by Plant Growth-Promoting Rhizobacteria on Potato Plant Development and Yield," Phytopathology 70: 1078-82 (1980a). PA1 Kloepper et al, "Enhanced Plant Growth by Siderophores Produced by Plant Growth-Promoting Rhizobacteria," Nature 286: 885-86 (1980b). PA1 Suslow & Schroth, "Rhizobacteria on Sugar Beets: Effects of Seed Application and Root Colonization on Yield," Phytopathology 72: 199-206 (1982). PA1 Olsen & Misaghi, "Responses of Guayule (Parthenium argentatum) Seedlings to Plant Growth Promoting Fluorescent Pseudomonads," Plant and Soil 77: 97-101 (1984). PA1 Caesar & Burr, "Growth Promotion of Apple Rootstocks by Rhizobacteria," Phytopathol. 75(11): 1290-91 (1985).
It is still unclear by what range of mechanisms PGPR increase plant growth. Work by Kloepper et al (1980b) on the role of microbial iron transport agents (siderophores) in the rhizosphere indicates one mechanism by which some fluorescent pseudomonad PGPR promote plant growth, i.e., by antagonism to potentially deleterious, indigenous fungi and bacteria. In this vein, it has been proposed that rhizobacteria-mediated enhancement of plant growth generally involves interactions of PGPR with rhizosphere microflora, possibly leading to the displacement of microorganisms detrimental to plant growth. For example, Canadian patent No. 1,172,585 discloses the use of particular strains of naturally-occurring pseudomonads to benefit plant growth in root crops by reducing the population of other indigenous root-zone microflora that adversely influence plant growth. Another hypothesis is that the PGPR elaborate substances that directly stimulate plant growth, such as nitrogen, hormones and compounds promoting the mineralization of phosphates.
The cited Canadian patent also illustrates the fact that the impact on growth of known PGPR strains is restricted largely to roots; hence, documented improvements in yield per se (as distinguished, for example, from simple root elongation) have been in root crops like radish, sugar beet and potato. In other agronomic crops, where an effect of root growth cannot be directly correlated with increased yield, there have been no reports of yield-related effects by PGPR. In the rubber-producing desert shrub called guayule, for example, PGPR-mediated increases in shoot weight, but not in seed yield, have been reported. In contrast, no rhizobacterial strains have been identified heretofore that enhance seed (grain) yield in any cereal or oilseed crop, or in maize.